I have checked it out, they don't make any claims like you would expect, if they really do better than flat vents.

I have one 14"rd. wind vent in a attic that is 800 Sq.' At a 7 mi. an hr. wind, it will move 4 times more than the 12" sq vent that was there before. This was
tested with a CFM tester that i use for balancing commercial kitchen in restaurants. Paul

Your right the wind moving over a hole can pull a vacuum, but the same wind turning the wind vane moves a lot more air. and on a no wind day most roof vents are 6 or 8', mine is 14" and because its a flat roof i put it two foot in the air. This creates more chimney
action and moves more air. also it does not get under the snow and stop working like flat vents. I really like the wind vents